ABSTRACT
We show that employing localized surface plasmon resonators to probe environmental media will always lead to dissimilar optical sensitivities to permittivity and permeability. We find that while the permittivity sensitivities of diverse plasmonic structures display a geometry-independent universal scaling relation, the permeability sensitivities are highly dependent on the metals' geometries and resonant modes. Similar results are also found in mixed real/spoof localized surface plasmon resonators, and the phenomena can be universally scaled to the normalized effective plasmon frequencies. Importantly, the results put a fundamental constraint for all plasmonic-assisted nonlinear magneto-optical phenomena, including the Faraday effect, magneto-optical Kerr effect, and Cotton-Mouton effect.
ABSTRACT
We establish experimental and numerical evidence that the refractive index sensitivities of various subwavelength plasmonic sensors obey a simple universal scaling relation that the sensitivities linearly increase with λm/neff (where λm is the resonant wavelengths and neff is the effective refractive index of the environment) and exhibit a slope equal to 1 instead of 2 predicted theoretically. The universal scaling relation is independent of the geometrical structures or contributions of multipolar resonances of individual metal structures (i.e. plasmonic atoms). It is also independent of spatial distributions or field-enhancements of electromagnetic hot spots in coupled metal structures (i.e. plasmonic molecules). The universal scaling relation reveals the fundamental standing wave resonances for all plasmonic atoms and the predominant near-field electric couplings for most plasmonic molecules. The established universal relation also helps to exclude some magnetically coupled plasmonic molecules for practical applications due to their reduced sensitivities.
